1. Field of the Invention
The present invention relates to optical disc systems for performing recording and/or reproduction of an information signal, and optical discs used in these systems.
2. Description of the Prior Art
An optical disc system shown in FIG. 1 performs recording in and/or reproduction from an optical recording medium, such as an optical disc.
A conventional optical disc system shown in FIG. 1 is arranged such that a disc-like, one-sided optical recording medium 50 is mounted and an optical system including a laser unit 58, an objective lens 59, and the like is arranged above the one-sided optical recording medium 50. The numerical aperture, referred to as NA hereinafter, of the objective lens 59 is set to fall within the range of 0.50 to 0.53.
In order to drive the optical system in focusing and tracking directions with respect to the optical recording medium 50, a drive system (not shown) is provided.
The one-sided optical recording medium 50 is formed as follows. A reflecting and recording layer 55 and a protective cover film 56 are sequentially stacked on one surface of a light-transmitting transparent substrate 51 made of, for example, polycarbonate. The transparent substrate 51 has a predetermined thickness t.sub.1. The thickness of a conventional substrate is set to be 1.2 mm.
An operation of the previously known disc system will be described below.
The one-sided optical recording medium 50 is placed on and driven by a rotary table (not shown). A laser beam emitted from the laser unit 58 is focused through the objective lens 59 onto the recording layer 55. A change in intensity of the focused laser beam in accordance with the data to be recorded produces pits in the reflective recording layer 55.
In order to cope with an increase in information volume, more information signals must be recorded in the recording layer of an optical disc, however, there are dimensional constraints on the optical disc and other constraints on the lens and light source that presently prevent such increases in information volume.